Vibrating conduit sensors, such as Coriolis mass flowmeters and vibrating densitometers, typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness, and damping characteristics of the conduit and the material contained therein.
It is well known to use vibrating flowmeters to measure mass flow and other properties of materials flowing through a pipeline. For example, vibrating Coriolis flowmeters are disclosed in U.S. Pat. No. 4,491,025 issued to J. E. Smith, et al. of Jan. 1, 1985 and also Re. 31,450 to J. E. Smith of Nov. 29, 1983. These flowmeters have one or more fluid tubes. Each fluid tube configuration in a Coriolis mass flowmeter has a set of natural vibration modes, which may be of a simple bending, torsional, radial, lateral, or coupled type. Each fluid tube is driven to oscillate at resonance in one of these natural modes. The vibration modes are generally affected by the combined mass, stiffness, and damping characteristics of the containing fluid tube and the material contained therein, thus mass, stiffness, and damping are typically determined during an initial calibration of the flowmeter using well-known techniques.
Material flows into the flowmeter from a connected pipeline on the inlet side of the flowmeter. The material is then directed through the fluid tube or fluid tubes and exits the flowmeter to a pipeline connected on the outlet side.
A driver, such as a voice-coil style driver, applies a force to the one or more fluid tubes. The force causes the one or more fluid tubes to oscillate. When there is no material flowing through the flowmeter, all points along a fluid tube oscillate with an identical phase. As a material begins to flow through the fluid tubes, Coriolis accelerations cause each point along the fluid tubes to have a different phase with respect to other points along the fluid tubes. The phase on the inlet side of the fluid tube lags the driver, while the phase on the outlet side leads the driver. Sensors are placed at two different points on the fluid tube to produce sinusoidal signals representative of the motion of the fluid tube at the two points. A phase difference of the two signals received from the sensors is calculated in units of time.
The phase difference between the two sensor signals is proportional to the mass flow rate of the material flowing through the fluid tube or fluid tubes. The mass flow rate of the material is determined by multiplying the phase difference by a flow calibration factor. The flow calibration factor is dependent upon material properties and cross sectional properties of the fluid tube. One of the major characteristics of the fluid tube that affects the flow calibration factor is the fluid tube's stiffness. Prior to installation of the flowmeter into a pipeline, the flow calibration factor is determined by a calibration process. In the calibration process, a fluid is passed through the fluid tube at a given flow rate and the proportion between the phase difference and the flow rate is calculated. The fluid tube's stiffness and damping characteristics are also determined during the calibration process as is generally known in the art.
One advantage of a Coriolis flowmeter is that the accuracy of the measured mass flow rate is largely not affected by wear of moving components in the flowmeter, as there are no moving components in the vibrating fluid tube. The flow rate is determined by multiplying the phase difference between two points on the fluid tube and the flow calibration factor. The only input is the sinusoidal signals from the sensors indicating the oscillation of two points on the fluid tube. The phase difference is calculated from the sinusoidal signals. Since the flow calibration factor is proportional to the material and cross sectional properties of the fluid tube, the phase difference measurement and the flow calibration factor are not affected by wear of moving components in the flowmeter.
A typical Coriolis mass flowmeter includes one or more transducers (or pickoff sensors), which are typically employed in order to measure a vibrational response of the flow conduit or conduits, and are typically located at positions upstream and downstream of the driver. The pickoff sensors are connected to electronic instrumentation. The instrumentation receives signals from the two pickoff sensors and processes the signals in order to derive a mass flow rate measurement, among other things.
Typical Coriolis flowmeters measure flow and/or density through the use of a coil and magnet as a pickoff sensor to measure the motion of a meter's vibrating flow tube/tubes. The mass flow rate through the meter is determined from the phase difference between multiple pickoff signals located near the inlet and outlet of the meter's flow tubes. However, it is possible to measure flow using strain gages in place of coil/magnet pickoffs. A fundamental difference between the two sensor types is that coil/magnet pickoffs measure the velocity of the flow tubes and strain gages measure the strain of the flow tubes which is proportional to the tubes' displacement. As such, the placement of each type of sensor will not necessarily be in the same location.
Strain gages have a number of advantages over coil/magnet pickoffs. Strain gages are cheaper to produce and implement than coil/magnet pickoffs. They also help to eliminate point masses that may adversely affect system operation. Additionally, strain gages do not need a reference point from where to measure strain like coil/magnet pickoffs. This allows for single flow tube designs that are not possible with coil/magnet pickoffs.
Although prior art attempts have been made to provide a means for utilizing strain gages instead of magnet/coil pickoffs for flowmeters, the practical applications of these attempts are relatively limited. A known problem is that strain gages have difficulty resolving strain of relatively thick metal flow tubes. In particular, Coriolis forces are barely discernable over background signals due to the extremely small tension and compression forces that result from fluid flow. Therefore changes in gage resistance are quite small and difficult to measure accurately. For this reason, prior art devices often rely on attaching projections from the flow tubes to measure strain changes in these associated structures, and not on the flow tube itself or using softer materials in construction that are more susceptible to strain. For example, U.S. Pat. No. 6,748,813 discloses strain gages attached to silicone arms that are connected to a flowmeter flow tube. Strain in the silicone is measured for flow calculations. By adding bars or protrusions to flow tubes, this creates an undesirable point mass that may affect meter performance. U.S. Pat. No. 7,500,404, on the other hand, discloses soft tubes for use as flow tubes, or rigid tubes that have soft regions that can experience greater levels of strain than ridged tubes. For sustained industrial applications, the use of soft tubes is not practical due to potentially high pressures, high temperatures, and caustic properties of fluids within the tubes. Soft materials such as silicone have drastically lower tensile strength as compared to stainless steel, for example—a preferred material for industrial flowmeter flow tubes.
Another problem with strain gages is that they are susceptible to DC drift, which makes a steady phase calculation difficult, and thus yielding inaccurate flow rate readings.
The embodiments described below overcome these and other problems and an advance in the art is achieved. The embodiments described below provide a flowmeter with strain gages to detect strain of the flow tube. By connecting various combinations of strain gages having varying placements and orientations on a flowmeter with various combinations of bridge circuits, signal amplification and background cancellation is accomplished. A high-pass filter is utilized to mitigate DC drift issues, while amplification and filter circuitry is disclosed that provides an apparatus and method to electrically process strain signals in the absence of pre-meter electronics digital signal processing. By processing signals with analog circuitry prior to input into meter electronics, strain gages may easily be adapted for use in flowmeter units that were originally designed for coil/magnet type pickoffs.